editors/nano: Make several port improvements

• Switch to https:// MASTER_SITES
• Use provided .xz for distribution file, saving: 50.5% / 1415kB
• Add LICENSE_FILE
• Remove CPPFLAGS/LDFLAGS variables, added 19 years ago when the port was first created. [1] They are incorrectly (globally) scoped, no longer necessary and incorrect for the ncurses:base case. Instead use Uses/ncurses.mk variables and provide these to configure to ensure includes/lib searches and linking are never mixed between base and ports.

[1] ports r29979

Approved by: danilo (maintainer)
MFH: 2019Q2 (blanket: bugfix)
Differential_Revision: D20660